In the simplest of the options, this does append to the pipe stream:
$ echo "750/12.5" | { bc; echo "/24"; }
60
/24
However that has an unexpected newline, to avoid that you need to either use tr:
$ echo "750/12.5" | { bc | tr -d '\n' ; echo "/24"; }
60/24
Or, given the fact that a command expansion removes trailing newlines:
$ printf '%s' $( echo "750/12.5" | bc ); echo "/24"
60/24
But probably, the correct way should be similar to:
$ echo "$(echo "750/12.5" | bc )/24"
60/24
Which, to be used in bc, could be written as this:
$ bc <<<"$(bc <<<"750/12.5")/24"
2
Which, to get a reasonable floating number precision should be something like:
$ bc <<<"scale=10;$(bc <<<"scale=5;750/12.5")/24"
2.5000000000
Note the need of two scale, as there are two instances of bc.
Of course, one instance of bc needs only one scale:
$ bc <<<"scale=5;750/12.5/24"
In fact, what you should be thinking about is in terms of an string:
$ a=$(echo "750/12.5") # capture first string.
$ echo "$a/24" | bc # extend the string
2
The comment about scale from above is still valid here.
echo $(echo "750/12.5" | bc )/24" | bc
?